The present invention relates to elastomer layers and a process for forming the elastomer layers, and more specifically, to fluorinated carbon filled elastomers useful as layers for electrostatographic members, especially xerographic members such as bias charging members, and methods thereof. In embodiments, there are selected fluorinated carbon filled elastomers which are useful as layers for components in electrostatographic processes, especially xerographic processes, including bias charging rolls, belts and other members, for example, bias charging belts, films and rolls, and the like. In embodiments, the present invention allows for the preparation and manufacture of bias charging members with superior electrical and mechanical properties, including controlled and uniform conductivity in a desired resistivity range, and increased mechanical strength, durometer, tensile strength, elongation and toughness. Further, in embodiments, the layers also exhibit excellent properties such as statistical insensitivity of conductivity to changes in temperature and humidity, intense continuous corona exposure, corrosive environments, solvent treatment, running time or cycling to high electric fields and back. Also, in embodiments, the layers permit a decrease in contamination of other xerographic components such as photoconductors. In addition, the present invention, in embodiments, allows for use of a single DC bias. Moreover, in embodiments, ozone contamination is decreased, and thus the biasable charging members are more environmentally friendly.
In a conventional charging step included in electrophotographic processes using an electrophotographic photosensitive member, in most cases a high voltage (DC voltage of about 5-8 KV) is applied to a metal wire to generate a corona, which is used for the charging. In this method, however, a corona discharge product such as ozone and NO.sub.x is generated along with the generation of the corona. Such a corona discharge product deteriorates the photosensitive member surface and may cause deterioration of image quality such as image blurring or fading or the presence of black streaks across the copy sheets. Further, ozone contamination may be harmful to humans if released in relatively large quantities. In addition, the photosensitive member which contains an organic photoconductive material is susceptible to deterioration by the corona products.
Also, as the power source, the current directed toward the photosensitive member is only about 5 to 30% thereof. Most of the power flows to the shielding plate. Thus, the efficiency of the charging means is low.
For overcoming or minimizing such drawbacks, methods of charging have been developed using a direct charging member for charging the photosensitive member. For example, U.S. Pat. No. 5,017,965 to Hashimoto et al, the subject matter of which is hereby incorporated by reference in its entirety, discloses a charging member having a surface layer which comprises a polyurethane resin. Also, European Patent Application 0 606 907 A1, the subject matter of which is hereby incorporated by reference in its entirety, discloses a charging roller having an elastic layer comprising epichlorohydrin rubber, and a surface layer thereover comprising a fluorine containing bridged copolymer.
These and other known charging members are used for contact charging for charging a charge-receiving member (photoconductive member) through steps of applying a voltage to the charging member and disposing the charging member being in contact with the charge-receiving member. Such bias charging members require a resistivity of the outer layer within a desired range. Specifically, materials with too low resistivities will cause shorting and/or unacceptably high current flow to the photoconductor. Materials with too high resistivities will require unacceptably high voltages. Other problems which can result if the resistivity is not within the required range include nonconformance at the contact nip, poor toner releasing properties and generation of contaminant during charging. These adverse affects can also result in that the bias charging members tend to have non-uniform resistivity across the length of the contact member. It is usually the situation that most of the charge is associated at or near the center of the charge member. The charge seems to decrease at points farther away from the center of the charge member. Other problems include resistivity that is susceptible to changes in temperature, relative humidity, running time, and leaching out of contamination to photoconductors.
Other factors affecting bias charging member performance include the use of AC and/or DC potential. Typically, an AC potential is normally used along with a DC "controlling potential" to aid charging control. The advantage of using AC lies in the reduction of surface contamination sensitivity. The use of AC creates a corona in the pre and post nip regions of the device so that the charging component related to charge injection in the nip is less important. This "injection component" is very sensitive to the surface properties of the materials and is a large factor for preventing charging non-uniformity which may occur when only DC is used.
However, the AC current required for operating the AC bias system is proportional to the process speed. This limits the application of bias devices to low speed machines. Also, the AC power supply is relatively expensive. Therefore, it is desirable from a cost and design standpoint to have a single DC bias system. This requires materials with an optimum and stable resistivity. Otherwise, use of a single bias will cause pre-nip breakdown, charging non-uniformity, and contamination.
Attempts at controlling the resistivity within the desired range have focused on controlling the resistivity range at the pre and post nip areas. These attempts have included adding ionic additives to the elastomer layers. European Patent Application 0 596 477 A2, the subject matter of which is hereby incorporated by reference in its entirety, discloses a charging member comprising at least an elastic layer comprising epichlorohydrin rubber and a surface layer disposed thereon, the surface layer comprising at least a semiconductive resin and an insulating metal oxide contained in the semiconductive resin. While addition of ionic additives to elastomers may partially control the resistivity of the elastomers to some extent, there are problems associated with the use of ionic additives. In particular, undissolved particles frequently appear in the elastomer which causes an imperfection in the elastomer. This leads to a nonuniform resistivity, which in turn, leads to poor transfer properties and poor mechanical strength. Furthermore, bubbles appear in the conductive elastomer, some of which can only be seen with the aid of a microscope, others of which are large enough to be observed with the naked eye. These bubbles provide the same kind of difficulty as the undissolved particles in the elastomer namely, poor or nonuniform electrical properties, poor mechanical properties such as durometer, tensile strength, elongation, a decrease in the modulus and a decrease in the toughness of the material. In addition, the ionic additives themselves are sensitive to changes in temperature, humidity, operating time and applied field. These sensitivities often limit the resistivity range. For example, the resistivity usually decreases by up to two orders of magnitude or more as the humidity increases from 20% to 80% relative humidity. This effect limits the operational or process latitude. Moreover, ion transfer can also occur in these systems. The transfer of ions will lead to contamination problems, which in turn, can reduce the life of the machine. Ion transfer also increases the resistivity of the elastomer member after repetitive use. This can limit the process and operational latitude and eventually, the ion-filled elastomer component will be unusable.
Conductive particulate fillers, such as carbons, have also been used in an attempt to control the resistivity. U.S. Pat. No. 5,112,708 to Okunuki et al., the disclosure of which is hereby incorporated by reference in its entirety, discloses a charging member comprising a surface layer formed of N-alkoxymethylated nylon which may be filled with fluorinated carbon. Generally, carbon additives control the resistivities and provide stable resistivities upon changes in temperature, relative humidity, running time, and leaching out of contamination to photoconductors. However, carbon particles disperse poorly in elastomers. Further, the required tolerance in the filler loading to achieve the required range of resistivity has been extremely narrow. This along with the large "batch to batch" variation leads to the need for extremely tight resistivity control. In addition, carbon filled elastomer surfaces have typically had very poor dielectric strength and sometimes significant resistivity dependence on applied fields. This leads to a compromise in the choice of centerline resistivity due to the variability in the electrical properties, which in turn, ultimately leads to a compromise in performance.
Therefore, there exists a specific need accomplished with the present invention in embodiments thereof for an elastomer outer surface for charging members which allows for a stable conductivity in the desired resistivity range without the problems associated with ionic additives and carbon additives.